Planck and the Planets

As Planck scans the microwave sky, looking out at the early Universe, it also sees things much closer to home. Now and again, a planet will appear in the path of Planck's beams as it spins around the sky. The planets move around the sky, and so don't always appear in the same place, and are normally observed only a couple of times a year. As of November 2009, Planck has seen Mars, Jupiter and Neptune. Saturn and Uranus will be later this year and early next year. Only the planets further from the Sun than the Earth can be observed, as Planck never points towards the Sun. At it's vantage point of L2, the Earth, Moon and Sun are always behind, protecting the sensitive detectors from their bright glow. This also means that Venus and Mercury are not observable either.

A pale red dot

The planets are much too small for Planck to see any detail. for example, Jupiter is just half an arcminute (1/120th of a degree) across (depending on how far away it is at the time), while the narrowest of Planck's beams on the sky are around 5 arcminutes (1/12th of a degree) wide. So Jupiter, for example, is only about 1/10th the size of a Planck beam - a small but not totally negligible fraction. The other planets appear smaller, because they are physically smaller and most are further away.

The planets are very bright since they are much warmer than the rest of the sky seen at microwave wavelengths, and so despite their small size they are still seen by Planck. At these wavelengths, Jupiter has a characteristic temperature of around 200K (-70oC), compared with the CMB which is only 3K (-270oC) and the Galaxy which is only up to about 50K (-220oC).

Calibration, Calibration, Calibration

To be able to work out anything about the early Universe, all of Planck's detectors must be calibrated. That means a number of things. Firstly, their sensitivity must be known so that the measurements can be turned into a physical number, such as the power detected or the temperature of the object. Secondly, the size of the beams must be well known as this can causes a "blurring" of the image which has to be taken into account correctly. Without knowing both of these very well, along with a whole bunch of other parameters describing the instrument, it is much harder to get at the key scientific results.

A varying light

The light Planck measures is partly reflected sunlight and partly microwave light emitted by the planets themselves. As the planets orbit the Sun, and move nearer and farther from the Earth, their brightness varies. This is also partly because their temperatures change with their own seasons and conditions - particularly Mars, which as well as having seasons also has dust storms which cover the planet now and again and change its "albedo" - the amount of sunlight it reflects. This means that the planets can be very tricky to use to calibrate the sensitivity of the detectors, though models are getting better and better all the time. For example, Jupiter's brightness is now known to around 1% thanks to WMAP observations, and since it doesn't really have any seasons to speak of it is fairly straightforward to predict.

The major use of the planets by Planck is to calibrate the size of the beams of Planck. By observing them as Planck spins on it axis, the size of the beams can be calculated by how bright the planets appears. It's complicated by the fact that the planets are not truly point-like, but are actually (very small) discs, but this can be taken into account.

Moving targets

A key requirement is knowing exactly where Planck is looking and exactly where the planets are in the sky. Luckily, Planck has star trackers which help keep track of the direction it's looking in, and the planets' orbits are known very well thanks to hundreds of years of careful observations by astornomers through the ages.

All of this information is put into the huge database of data, and analysed accordingly. Using the key information provided by observations of the planets, along with other calibration sources such as the Crab Nebula, Planck scientists can get a better understanding of what the rest of the data means and therefore a more accurate picture of the Universe.